1. Field of the Invention
The present invention relates generally to methods and apparatus for cleaning substrates (e.g., coated and/or uncoated substrates) and, in one embodiment, to a method and apparatus for cleaning a glass substrate having a photoactive and/or hydrophilic surface.
2. Technical Considerations
For many substrates, e.g., glass substrates such as architectural windows, automotive transparencies, and aircraft windows, it is desirable for good visibility that the surface of the substrate have minimal accumulation of surface contaminants, such as common organic and inorganic surface contaminants, for as long a duration as possible. In order to reduce the accumulation of organic surface contaminants, a hydrophilic and/or photoactive (“PA”) coating can be deposited on the glass surface. The terms “photoactive” or “photoactively” refer to the photogeneration of an electron-hole pair when illuminated by activating radiation in a particular frequency range. The activating radiation can be in the ultraviolet (“UV”) or visible ranges of the electromagnetic spectrum. By “ultraviolet range” is meant electromagnetic radiation in the range of 280 nanometers to less than 395 nanometers. By “visible range” is meant electromagnetic radiation in the range of 395 nm to 800 nm. Above a certain minimum thickness, these PA coatings are typically photocatalytic (“PC”). By “photocatalytic” is meant a surface, such as a coating, having some degree of self-cleaning properties. By “self-cleaning” is meant a surface or coating which upon exposure to electromagnetic radiation in the photoabsorption band of the material interacts with organic contaminants on the surface to degrade or decompose at least some of the organic contaminants. As the coating thickness decreases, photocatalytic activity can be difficult to measure. In addition to their self-cleaning properties, these PC coatings can also be hydrophilic, e.g., water wetting with a contact angle with water of generally less than 20 degrees. The hydrophilicity of the PC coatings helps reduce fogging, i.e., the accumulation of water droplets on the coating, which fogging can decrease visible light transmission and visibility through the coated substrate.
While these photoactive coatings can provide a glass substrate, such as a window, with improved low maintenance properties, such as some self-cleaning properties with regard to organic contaminants, they do not generally decompose inorganic contaminants. Therefore, since typical household water contains dissolved minerals and inorganic ions (such as magnesium, calcium, iron, and/or sodium ions and compounds), when household water is used to clean a substrate having a photoactive coating, upon evaporation of the water the dissolved minerals and inorganic materials can deposit on the coating surface, typically as inorganic salts. These inorganic salts can form as crystals. In the event that hydrophobic areas or spots form on the surface, these crystals can accumulate on the hydrophobic spots. Conventional photoactive, e.g., photocatalytic, coatings are ineffective in breaking down these inorganic solids. A problem with these inorganic solids is that as they accumulate, they can decrease visible light transmission through the window and/or make the window appear hazy. Additionally, these inorganic solids can form visible streaks or patterns on the window. This streaking and patterning is believed due to the uneven distribution of the minerals and inorganic salts that arises from the sheeting action of the photoactive, e.g., hydrophilic, surface. The overall effect of these inorganic materials is to make the window appear hazy or streaky over time. Additionally, silicates in the water used to clean the window can permanently bond to exposed areas of the glass surface and can also make the window appear hazy.
Therefore, it would be advantageous to provide a method and/or device to clean a substrate, such as a glass surface having a photoactive and/or hydrophilic surface or coating, which reduce or eliminate at least some of the drawbacks described above.